PayPal actually stopped caring about adult content a while back; if you comb through the ToS it doesn't even mention it any more. As a result, it gets used in more places, more money is made, and nobody actually cares.
The method being used here is pretty placeholder, and I have to admit that adding an extra 2-3 steps to the process probably limits how often people are going to want to get in on it. On the other hand, the fact that this is just quick little donations *anyway* means that I don't think anyone who wants to give money is going to be perturbed by it.